The present invention relates to a PWM (Pulse Width Modulation) type D/A (digital-to-analog) converter used in demodulating, for example, a PCM (Pulse Code Modulation) audio signal.
In demodulation (D/A converting) a PCM audio signal by using a PWM type D/A converter (hereinafter called a PWM converter), it is necessary to modulate an audio signal into PWM waveforms symmetrical in time relative to certain timing point in order to avoid generating a phase error in a demodulated signal.
FIG. 1 shows the waveforms of output signals according to to a conventional PWM type D/A converter.
In FIG. 1, seven values from +3 to -3 are the values of digital input signals, T is the period (sample period) for input signals, and t is the period of a clock signal CK used for conversion. The resolution m of the conventional of PWM signals is given by: EQU m=T/(2t)-1 (1)
As seen from FIG. 1, since T/t=16, the resolution m becomes 7 from the equation (1). PWM signals corresponding to respective seven values +3 to -3 can therefore be obtained.
As described above, if a conventional PWM converter is used in demodulating PCM audio signals at high speed and with high resolution, it is necessary, as understood from the equation (1), to use a clock signal of very high frequency. The use of such a clock signal poses various problems such as an increase of unnecessary radiation which interferes other devices, and a difficulty in obtaining a stable oscillator element.